possum:
The thrust imbalance between the SRB's, ET and Shuttle is carried by the ET forward thrust beam. It runs through the ET from SRB to SRB. The attach points are very strong, made of Inconel I believe, as are a lot of high stress parts. Not really that hard to believe if you think about 2.75 million lbs. shear on each side it would take a 3.5 inch diameter shaft made of material capable of 150,000 psi in shear to take that load.

The two "mini-towers" on the MLP are called Tail Service Masts (TSM) and they are the LOX and LH2 umbilicals that fuel the vehicle prior to lift-off. The LOX and LH2 go into the Orbiter aft propulsion piping and into the ET and is drained back through the same piping to the engines during launch. The TSM's are attached while in the VAB and they are designed to remain attached to the Oribiter with significant movement, not just during rollout but also during main engine buildup prior to SRB ignition. The TSM's must remain attached until the SSME's are at full thrust and the SRB's iginite (they actually separate "in flight") because if the SSME's shut down before reaching full thrust, you have to be able to drain the ET and safe the vehicle.

Jamie Young:

--- Quote ---rmathews3 - 11/1/2006 7:23 PM

The two "mini-towers" on the MLP are called Tail Service Masts (TSM) and they are the LOX and LH2 umbilicals that fuel the vehicle prior to lift-off. The LOX and LH2 go into the Orbiter aft propulsion piping and into the ET and is drained back through the same piping to the engines during launch. The TSM's are attached while in the VAB and they are designed to remain attached to the Oribiter with significant movement, not just during rollout but also during main engine buildup prior to SRB ignition. The TSM's must remain attached until the SSME's are at full thrust and the SRB's iginite (they actually separate "in flight") because if the SSME's shut down before reaching full thrust, you have to be able to drain the ET and safe the vehicle.--- End quote ---

Thanks, that explains why they are on the MLP, so they can keep the Shuttle steady from the point of mating to launch. Seems obvious now, but sometimes you need someone to tell you! :)

Orbiter Obvious:

--- Quote ---rmathews3 - 11/1/2006 7:23 PM

The two "mini-towers" on the MLP are called Tail Service Masts (TSM) --- End quote ---

--- Quote ---Jamie Young - 11/1/2006 7:42 PMThanks, that explains why they are on the MLP, so they can keep the Shuttle steady from the point of mating to launch. Seems obvious now, but sometimes you need someone to tell you! :)--- End quote ---

The TSM's don't really keep the Shuttle steady, if fact, you don't want them to impart any loads into the Shuttle structure. The Shuttle stack is supported entirely from the base of the SRB's, with the Shuttle cantilevered off the side of the ET. The TSM attachment "floats" with the Shuttle as is sways back and forth. In fact, when the SSME's are building thrust prior to SRB ignition, the Shuttle stack is flexed forward and when it rocks back to vertical, the SRB's are fired. The TSM's must stay attached to the Shuttle during this "twang" of rocking back and forth. If you look at the video of the STS-105 launch on page 5 of the FTP video collection, you can see the aft end of the Shuttle lift up more than a foot when the engines ignite, this translates into 30 inches of movement at the tip of the ET. If you look real close, you can see the TSM umbilical plate attached to the Shuttle right next to the OMS pod and when the stack rocks back and the SRB's ignite, the umbilical plate disappears into the TSM in the blink of an eye.

By the way, those "sparklers" you see during SSME ignition are called Radially Outward Firing Initiators (ROFI's) and they are there to ignite any unburned hydrogen coming out of the SSME's during startup, otherwise hydrogen gas could build up in pockets of dead air in the aft area and explode.

The TSM's are attached to the Shuttle to provide services, the main service being LOX and LH2 fill and drain. There are also many electrical and fluid services going through the umbilical plate. The mating plate is about 3 feet wide by 4 feet tall and it has numerous quick disconnects (QD's) for fluids and electrical services. Many things go through this interface including all of the software commanding up until T-31 seconds when control of the countdown is handed off to the Shuttle. If the launch team wants to stop the countdown, the commands go through this interface. That is another reason why these umbilicals are attached until the SRB's are ignited and there's no stopping the launch. All the critical interfaces between the ground and the flight vehicle go through the TSM's. And since there are 2 TSM's, there are redundant services going through each TSM so that if one inadvertently disconnects prematurely, you still have the critical commanding capability through the other one.

psloss:
Regarding the tail service masts, there are good OTV engineering cameras on the MLP "deck" that usually focus on the T-0 umbilicals that run from them into the orbiter; as you note the twang effect is very noticeable there.

I believe the hydrogen burn igniters start at T-10 seconds and stop at T-0 (don't know if that's at depletion or not); what's interesting about that is the last cutoff on the pad (STS-68) was initiated so close to T-0 (at -1.9 sec.), that the igniters stopped before the shutdown sequence was completed.

There have also been a few cases where a hold was called after the igniters started but before the engine start commands were sent (STS-93 being one example; another would be the first STS-26 FRF attempt).